Organic electroluminescene display

ABSTRACT

An organic EL display equipped with an organic EL device with an organic material sandwiched by at least two electrodes, including: a light emitting layer which emits light; a front reflecting portion arranged on the side of a viewer with respect to the light emitting layer; and a rear reflecting portion arranged on the side opposite to the viewer with respect to the light emitting layer, wherein the optical film thickness of the organic material, intensity reflectance R 1  at the front reflecting portion and intensity reflectance R 2  at the front reflecting portion are adjusted so that the external light intensity reflectance of the display viewed from the viewer is 10% or less by an optical interference effect.

[0001] The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2002-290519 filed on Oct. 2, 2002, whichis incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an organic electroluminescencedevice.

[0004] 2. Description of the Related Art

[0005] Generally, an organic electroluminescence device (hereinafter,referred to simply as “organic EL device”) has a structure with anorganic material laminate sandwiched between a transparent conductivefilm and a metallic electrode. The light (hereinafter, referred to as“EL light”) emitted within the device is extracted externally from thedevice through a transparent electrode.

[0006] The metallic electrode (backside electrode) arranged on a sidewhere the EL light is to be extracted, which is the side opposite to aviewer, is made of a metal alloy such as an alloy of Mg and Ag or analloy of Al and Li. The metal alloys having reflectance stronglyreflects the external light which is incident from the outside.Therefore, the display thus obtained is largely influenced by theexternal light reflected at the backside electrode, and provides poorvisibility like a mirror face when seen from a viewer.

[0007] In order to avoid such inconvenience, for the purpose of blockingthe reflection of the external light, an expensive circularly-polarizingfilter is generally attached to the exterior of the device. However, ina so-called “top emission structure” in which a light extractingelectrode is arranged on an organic film but not on the side of asubstrate, it is difficult to provide the circularly-polarizing filterto be in contact with the exterior of the device.

[0008] Even where the light extracting electrode is arranged on the sideof the substrate, in a case where the substrate itself is made thin, thethickness of the circularly-polarizing filter hinders low-profiling andweight reduction. Further, the transmittance of thecircularly-polarizing filter is about 40% so that the efficiency of theorganic EL light is half or less the case with no filter.

[0009] Another known technique for preventing reduction in thevisibility is to make the rear surface of the device non-reflective. Aknown method of making a non-reflective electrode is to use an absorbentmaterial as an electrode. Japanese Patent No.2529741 discloses a methodfor manufacturing a device with low reflectance having a backsideelectrode in a laminated structure as shown in FIG. 4 using opticalinterference.

[0010] Still another known technique for preventing the reduction in thevisibility is to make both electrodes transparent or semi-transparentand arrange a non-reflective film outside either electrode to suppressthe reflection of external light at the backside electrode, therebyimproving visibility.

[0011] The devices, in which the rear surface of the device is madelow-reflective or non-reflective to improve visibility without providinga circularly-polarizing filter, provide an abrupt reduction in theefficiency of externally extracting light from the device.

[0012] The internally emitted light in the organic EL device is emittedwith equal intensities in all the directions as shown in FIG. 1.Therefore, the light with the intensity equal to that emitted forward isalso emitted in the rear direction. The light actually extracted outwardfrom the device is the sum of electric field amplitudes of the lightemitted forward and the light emitted in the rear direction andreflectedby thebackside electrode. Generally, in the organic EL device,the thickness of an optical film in a laminated device structure isoptimized so that the phase difference between the forward emitted lightand the reflected light of the rearward emitted light satisfies thecondition of mutual intensifying of optical interference, therebyeffectively extracting the light multiply reflected from the backsideelectrode and other reflecting planes as EL light emitted outward.

[0013] When the non-reflective electrode is used as the backsideelectrode, the EL light emitted to the rear side is not extracted on thefront side. As a result, the intensity of the organic EL light thusexternally extracted is supposedly about ½ of that of the entire ELlight.

[0014] However, the actual experiment shows that the intensity of theorganic EL light externally extracted has been reduced to about ¼ in thedevice equipped with the non-reflective electrode as compared with theordinary EL device. This means that the efficiency is referred to abouthalf of that of ordinary device equipped with the circularly-polarizingfilter.

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the invention to provide a method ofimproving the intensity of externally extracted electroluminescencelight when using the technique of improving the visibility of thedisplay by reducing the reflectance on a rear side, in an organicelectroluminescence device using the light extracted to at least the oneside, and an organic EL display using such a technique.

[0016] In order to achieve the object, according to one aspect of theinvention, there is provided an organic electroluminescent displayincluding: an organic electroluminescent device having a light emittinglayer made of an organic material and at least two electrodessandwiching the light emitting layer; a front reflecting portionarranged on a side of a viewer with respect to the light emitting layer;and a rear reflecting portion arranged on a side opposite to the viewerwith respect to the light emitting layer, wherein the optical filmthickness of the light emitting layer, intensity reflectance R₁ at thefront reflecting portion and intensity reflectance R₂ at the rearreflecting portion are configured so that an intensity reflectance ofthe external light viewed from the viewer is set to be 10% or less by anoptical interference effect.

[0017] According to a second aspect of the invention, there is providedan organic electroluminescent display equipped with an organicelectroluminescent device having a laminated optical structure with lowreflectance and transmittance, wherein the organic electroluminescentdevice includes: a substrate; a first semi-transparent film; a secondsemi-transparent film; and a reflective film, and wherein the firstsemi-transparent film, the second semi-transparent film and thereflective film are laminated on the substrate in this order or an orderopposite thereto.

[0018] According to a third aspect of the invention, there is providedan organic electroluminescent display including: a laminated opticalstructure with low reflectance and transmittance; and an organicelectroluminescent device, wherein the organic electroluminescent deviceincludes: an organic electroluminescent layer having a light emittinglayer made of an organic material and a transporting layer configured totransport charges to the light emitting layer, and two electrodesconfigured to sandwich the organic electroluminescent layer, wherein thelaminated optical structure includes at least two layers, and whereinthe laminated optical structure includes the transporting layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above objects and advantages of the present invention willbecome more apparent by describing in detail preferred exemplaryembodiments thereof with reference to the accompanying drawings,wherein:

[0020]FIG. 1 is a view showing a sectional structure of an organic ELdevice in an organic EL display and a distribution of the intensity ofEL light emitted internally within the organic EL device;

[0021]FIG. 2 is a view for explaining an optical interference within thedevice of the light emitted from an internal EL emitting point;

[0022]FIG. 3 is a view showing the method for optically optimizing theorganic EL device structure;

[0023]FIG. 4 is a view showing a non-reflective laminated structure of aplurality of stacked optical films;

[0024]FIG. 5 is a graph showing the wavelength and reflectance of anon-reflective laminated structure consisting of aluminum and organicmaterial;

[0025]FIG. 6 is a graph showing the voltage-current characteristic ofeach of the organic EL device having the non-reflective laminatedstructure (Example 1) and organic EL device with no non-reflectivelaminated structure (Example 2);

[0026]FIG. 7 is a graph showing the current-brightness characteristic inthe Examples 1 and 2;

[0027]FIG. 8 is a conceptual view of the optical model of an organic ELdisplay;

[0028]FIG. 9 is a view illustrating the multiple reflection of the lightemitted internally in the device and the electric-field amplitude of thelight externally extracted from the device in the configuration of FIG.8;

[0029]FIG. 10 is a conceptual view of the multiple interferencephenomenon in the organic EL device about incident external light;

[0030]FIGS. 11A and 11B are views showing the inversion state of thephase of light;

[0031]FIG. 12 shows the distribution of brightness efficiency and curvecalculated by Equation (9);

[0032]FIG. 13 is a view showing an organic EL device according to afirst embodiment of the invention;

[0033]FIG. 14 is a view showing an organic EL device according to asecond embodiment of the invention;

[0034]FIG. 15 is a view showing an organic EL device according to afirst modification of the second embodiment;

[0035]FIG. 16 is a view showing an organic EL device according to asecond modification of the second embodiment;

[0036]FIG. 17 is a view showing an organic EL device according to athird modification of the second embodiment;

[0037]FIG. 18 is a view showing an organic EL device according to afourth modification of the second embodiment;

[0038]FIG. 19 is a view showing an organic EL device according to afifth modification of the second embodiment; and

[0039]FIGS. 20A and 20B are views showing an organic EL device accordingto a fifth modification of the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Referring now to the accompanying drawings, there are shownpreferred embodiments of the invention.

[0041] The inventor of the present invention, on a basis of anexperiment, have found that when the reflectance of the backsideelectrode of an organic electroluminescence device (hereinafter,referred to simply as “organic EL device”) is reduced nearly to zero,the efficiency of extracting an EL light externally from the device isreduced to approximately ¼, and the efficiency can be improved when thereflectance of the backside electrode is being effectively left.

[0042] The inventor of the present invention investigated, on a basis ofinterference model, the EL light emitted internally within a device andreflected light of external light which is incident from the outside ofthe device and reflected to the outside again, and found that both lightbeams are correlated with the reflectance and transmittance of each oftwo areas sandwiching an EL light-emitting layer within the device andan optical distance of each of the light beams within the device.

[0043] The inventors found that there is a range of the reflectance andtransmittance of each of the two areas sandwiching the above lightemitting layer which can improve the efficiency of externally extractingthe EL light while maintaining the reflectance of the external light ata low value, in view of the fact that a difference exists between theequation representative of the efficiency of extracting the EL light andthe equation representative of the reflectance of the external light.

[0044] Now referring to the drawings, a detailed explanation will begiven of an organic EL display.

[0045]FIG. 1 is a view showing a sectional structure of the organic ELdevice in an organic EL display and a distribution of the intensity ofthe EL light emitted internally within the organic EL device. Theorganic EL light device includes a transparent electrode 2 arranged on aglass substrate 1 and configured as an anode, a backside electrode 3configured as a cathode and an organic layer 4 sandwiched between thetransparent electrode 2 and the backside electrode 3.

[0046] A viewer 7 sees the EL light 8 emitted externally from the devicethrough the transparent electrode 2.

[0047] The organic layer 4 is made by heating and evaporating aplurality of organic materials within a vacuum chamber after thetransparent electrode 2 deposited on the glass substrate 1 has beensubjected to necessary steps such as patterning. After a holetransporting layer 4a having a thickness of several tens to severalhundreds nanometers has been deposited on the transparent electrode 2, alight emitting layer 5 is deposited thereon and an electron transportinglayer 4 b is further deposited thereon. As the case may be, one of theselayers has plural functions. The laminated structure of a plurality ofmaterials may have a single function. The above technique is directed tothe method of depositing low-molecular organic EL materials. But,solutions of polymer organic EL materials may be applied successivelyusing the technique of spin-coating or ink jetting. The deposition ofthe backside electrode 3 is a final step of the deposition and requiredto minimize the damage to the organic layer 4 previously deposited. Forthis purpose, in many cases, metal is heated and evaporated in vacuum.

[0048] The organic EL device consisting of a plurality of layers stackedsuccessively behaves as a PN-junction type semiconductor light-emittingdiode. Specifically, when a voltage is applied between the transparentelectrode 2 and the backside electrode 3, holes are injected from thetransparent electrode 2 serving as an anode and electrons are injectedfrom the backside electrode 3 serving as a cathode. The holes aretransported through the hole transporting layer 4 a and the electronsare transported through the electron transporting layer 4 b. Bothcarriers are re-combined within the light emitting layer 5. The energythus generated excites the molecules within the light emitting layer sothat when the excited molecules return to their ground state,fluorescence or phosphorescence is emitted.

[0049] Actually, because of the difference in the energy level at theinterfaces among the hole transporting layer 4 a, light-emitting layer 5and electron transporting layer 4 b, the carriers are concentrated tothe interfaces. In most devices, the carrier recombination occursintensively at either interface of the light emitting layer 5.Therefore, as seen from FIG. 1, it is known that the intensity of the ELlight within the device has an abrupt peak in the vicinity of theinterface of the light emitting layer 5, and is distributed so as todecrease exponentially toward the inside of the light emitting layer.

[0050] It is supposed that the distance until the peak of the intensitydecreases to 1/e is 5 nm to 20 nm. Most of EL light emitting points 6center on the vicinity of the interface to constitute a layer.

[0051]FIG. 2 is a view for explaining the optical interference withinthe device of the light emitted from the EL light emitting point 6. TheEL light emitted from the EL light emitting point 6 is isometric andnon-polarized, and is also emitted toward the backside electrode withthe intensity equal to that emitted toward the viewer 7. Each of the ELlight emitting points 6 which center on the interface of the lightemitting layer 5 emits electromagnetic waves which are not correlatedwith one another. The electromagnetic waves which are not correlatedwith one another are difficult to interfere with one another. However,in the organic EL device, strong interference occurs for the followingreason.

[0052] Generally, a lifetime of a fluorescent light of an organicmolecule is several nanoseconds. The fluorescent light propagatesseveral tens of centimeters in vacuum within the lifetime. Generally,the distance between the light emitting point 6 and the backsideelectrode 3, which is reflective, is several tens to several hundreds ofnanometers, and the refractive index of the organic material 4 is inrange of from 1.6 to 2. For this reason, the propagating distance of thelight reflected from the backside electrode 3 is sufficiently shorterthan the coherence length.

[0053] Therefore, when the light emitted by the single light emittingpoint 6 is reflected by the backside electrode 3 to return to the lightemitting point 6 again, the light emitting point 6 is continuing to emitthe electro-magnetic wave correlated with the reflected light. Each ofall the light emitting points brings about the interference phenomenondue to its own reflected light. The synthesized wave thus obtained isobserved as the EL light. As a result, the EL light internally emittedis affected by the self-interference phenomenon.

[0054]FIG. 3 is a view showing the method for optically optimizing theorganic EL device structure. In order to increase the light 8 extractedexternally from the device using the optical interference within thedevice, the thickness of each of the layers 4 a, 5 and 4 b constitutingthe organic material 4 is adjusted so that all the waves shown in FIG. 3are in phase.

[0055] More specifically, the optical distance lOa between the lightemitting interface 5 a and the rear reflecting plane 11 is adjusted sothat first forward emitted light 9 a and first rearward reflected light9 b are in phase. Namely, the optical distance lOa may be adjusted sothat a phase difference is integer times of 2 p between the round strokewhere the first rearward reflected light 9 b travels and the singlereflection stoke on the rear reflecting plane 11.

[0056] Likewise, the optical distance 10 b between the light emittinginterface 5 a and the front reflecting plane 12 is adjusted so thatfirst forward emitted light 9 a and first front reflected light 9 c arein phase. At this time, the first forward reflected light 9 c is firstreflected from the front reflecting plane 12, successively reflected bythe rear reflecting plane 11 and thereafter interferers with the firstfront emitted light 9 a.

[0057] Assuming that the distance 10 a between the light emittinginterface 5 a and the rear reflecting plane 11 is adjusted as describedabove, as long as the reflected light 9 d on the front reflecting planeof the first forward reflected light and the first rearward emittedlight 9 e are in phase, the first front emitted light 9 a and the firstfront reflected light 9 c are automatically in phase. Namely, theoptical distance 10 b may be adjusted so that a phase difference isinteger times of 2 p between the round stroke where the reflected light9 d on the front reflecting plane of the first forward reflected lighttravels and the single reflection stroke on the front reflecting plane12.

[0058] In the device with the optical distances 10 a and 10 b thusadjusted, the entire light internally emitted within the device shown inFIG. 3 is intensified in the same phase. It will be understood that onthe basis of the light emitting theory of the device, the process ofinjection, transportation and recombination of the carriers does notvary according to the thickness of the layers constituting the deviceand the distribution of the intensity of the emitted light within thedevice and the intensity itself are maintained. In other words, evenwith such adjustment of the optical film thickness, the intensity of theemitted light when the same current is passes through the device isalways the same.

[0059] In the organic EL display according of embodiments describedlater, the optical film thickness of the organic layer 4, the lightintensity reflectance R1 at the area from which the light nearer thanthe light emitting layer 5 to a viewer is reflected and the lightintensity reflectance R₂ at the area from which the light farther thanthe light emitting layer 5 from the viewer is reflected are adjusted sothat the external light intensity of the organic EL display becomes 10%or less owing to the optical interference effect. The reason will beexplained as follows.

[0060] The inventors of the present invention attempted an experiment,as shown in FIG. 1, of making the backside electrode 3 non-reflective.In the experiment using the technique described in Japanese PatentNo.2529741, there can be provided a non-reflective electrode using theoptical interference.

FIRST EXAMPLE

[0061] The technique described in Japanese Patent No.2529741 is asfollows. As seen from FIG. 4, on a glass substrate 13, a structure ismade in which a second semi-transparent layer 15 is sandwiched between afirst semi-transparent layer 14 and a highly reflective layer 16.

[0062] In the structure above, the reflectance and transmittance of thefirst semi-transparent layer 14, the thickness of the secondsemi-transparent layer 15 and the reflectance of the highly reflectivelayer 16 are adjusted so that the reflected light 18 a from the firstsemi-transparent layer 14 and the reflected light 18 b from thehighly-reflective layer 16 cancel each other, thereby making anon-reflective laminated structure. In a simple case, the secondsemi-transparent layer 15 may have an optical thickness of about ¼λ ofan objective wavelength.

[0063] For example, the first semi-transparent layer 14 and the highlyreflective layer 16 may be made of a metallic aluminum film. The secondsemi-transparent layer 15 can be made of a semi-transparent organicevaporated film. In addition, the second semi-transparent layer 15 maybe made of aluminum quinolinol complex (Alq₃) as an organic EL material.The refractive index of Alq₃ can be measured by ellipsometry technique,for instance, and is 1.76 at a wavelength of 525 nm.

[0064] Assuming that the refractive index is value n, and the thicknessis value d, the optical thickness is represented by “n×d”. In a case ofthe Alq₃ film, the thickness d which equals to ¼λ (when λ525 nm) can becalculated as follows.

n·d=525/4=131.25

[0065] Therefore, d=74.6 nm.

[0066] The reflectance was measured from the side of the glass on thestructure in which a very thin aluminum as the first semi-transparentlayer 14 is deposited on the glass substrate 13 by heating in vacuum,the Alq₃ film having a thickness of 80 nm as the second semi-transparentlayer 15 is deposited by heating in vacuum, and an aluminum film havinga thickness of 100 nm as the highly reflective layer 16 isvacuum-deposited. The measurement result is shown in FIG. 5. There canbe seen from FIG. 5 that the reflectance can be reduced to 1% at thelowest.

[0067] Further, as shown in FIG. 5, the lowest reflectance can beadjusted by adjusting the thickness of the Alq₃ film as the secondsemi-transparent layer 15. The reflectance can also be adjusted byvarying the thickness of the thin aluminum film as the firstsemi-transparent layer.

[0068] The non-reflective laminated structure thus obtained was used asa cathode of the organic EL device. The transparent electrode depositedon soda lime glass was patterned and the organic EL device was madethereon. A copper phthalocyanine film having a thickness of 25 nm as thehole injecting layer, an α-NPD film having a thickness of 45 nm as thehole transporting film and the Alq₃ film having a thickness of 60 nm asthe light emitting layer were successively vacuum-deposited. The filmthickness of each layer was selected in the combination which gives themaximum EL light intensity externally output by the experiment for theoptimum designing as shown in FIG. 3. Thereafter, after an Li₂O filmhaving a thickness of 0.3 nm as an electron injecting additive has beendeposited, the organic EL device was manufactured which has thenon-reflective laminated structure with the above thin aluminum, Alq₃ 80nm thick and highly reflective aluminum deposited.

SECOND EXAMPLE

[0069] In comparison to the above organic EL device, likewise, on thesoda lime glass equipped with ITO (Indium Tin Oxide), copperphthalocyanine 25 nm thick, α-NPD 45 nm thick and Alq₃ 60 nm thick weresuccessively vacuum-deposited. After Li₂O 0.3 nm thick as an electroninjecting additive has been deposited, the organic EL device wasmanufactured which does not have the non-reflective laminated structurewith aluminum 100 nm thick deposited.

[0070]FIG. 6 is a graph showing the voltage-current characteristic ofeach of the organic EL device having the non-reflective laminatedstructure of the first example, and organic EL device with nonon-reflective laminated structure of the second example. As can be seenfrom FIG. 6, the voltage-current characteristics in both devices arealmost the same. Therefore, it can be supposed that in both devices, theorganic diode equally functions and the internal light-emittingintensities for the same current are equal.

[0071]FIG. 7 is a graph showing the current-brightness characteristic inthe first and the second examples. As can be seen from FIG. 7, in allthe current ranges, the current-brightness efficiency in the firstexample is always about ¼ of that in the second example.

[0072] As understood from the above description, the organic EL devicein the second example is designed so that the rearward emitted lightintensifies the forward light using the reflection/interferencephenomenon. In the organic EL device in the first example, because therearward reflected light is not reflected forward, the intensity of theextracted light is supposedly about ½. However, a simple modelcalculation shows that the intensity is not ½, but about ¼.

[0073] This can be explained with reference to the followinginterference model within the organic EL device. First, opticalinterference phenomenon must be analyzed taking an incident angle oflight into consideration. The following explanation will be limited tothe case of vertical incidence. If the incidence angle is not vertical,the phase and reflection intensity vary according to the incidenceangle, but great importance may be given to the vertical incidence.Further, as regards the polarization of light, because the EL emittedlight is non-polarized, and distinction in the polarization in the caseof vertical incidence is not required, the polarization of light will bealso disregarded hereafter.

[0074] In the multiple interference in the optical film, the phase delayor attenuation due to absorption in the multiple reflection andpropagation must be taken into consideration. However, as long as theorganic EL device is optimized so that all the reflected beams of lightare in phase as in Example 1, all the beams of light externallyextracted from the device can be regarded to be in the same phase, andtherefore, the term of the phase will be omitted in the followingexplanation.

[0075] Further, where these optical interferences are explainedquantitatively, after all the superpositions of the electromagneticwaves have been considered, finally, the square of the absolute valuemust be taken as the intensity of light. Specifically, thecharacteristics such as intensity, reflectance and transmittance to bemeasured are represented by the square of amplitude of the electricfield of a light wave. Actually, the electromagnetic wave is representedby a complex number, and its square of the absolute value represents theintensity of light. However, for simplicity of explanation, theexplanation will be made in the region of real numbers since itsgenerality is not lost.

[0076] The formulas of Fresnel on the reflection/refraction phenomenonis described in a document “Principles of Optics I” by Max Born and EmilWolf, TOKAI UNIVERSITY PUBLISHER, 1974, on pages 61-73.

[0077] For example, the reflectance R measured on a certain reflectingplane denotes the reflectance of the intensity of light and is relatedwith a electric-field amplitude reflectance r as follows.

R=|r| ²   Equation (1)

Therefore, r={square root}(|R|)   Equation (2)

[0078] The relationship shown in the equations also holds for complexreflectance. In the following explanation, it is assumed that theintensity of reflectance and intensity of transmittance are denoted bycapital letters R and T, and the electric-field amplitude reflectanceand electric-field amplitude transmittance are denoted by small lettersr and t.

[0079] As described in the document “Principles of Optics I”, theelectric-field amplitude reflectance r and electric-field amplitudetransmittance t of the light which is incident from the medium withrefractivity n₁ to the medium with refractivity with n₂ are expressed asfollows.

r=(n ₁ −n ₂)/(n ₂ +n ₁)   Equation (3)

t=2·n ₁/(n ₂ +n ₁)   Equation (4)

[0080] As regards the same media, conversely, where the light isincident from the medium with n₂ to the medium with n₁, theelectric-field amplitude reflectance r′ and electric-field amplitudetransmittance t′ are expressed as follows.

r′=(n ₂ −n ₁)/(n ₂ +n ₁)=−r   Equation (5)

t′=2·n ₂/(n ₂ +n ₁)   Equation (6)

[0081] The corresponding intensity reflectance R, R′ and intensitytransmittance T, T′ are expressed as follows. $\begin{matrix}\begin{matrix}{R = {{r}^{2} = r^{2}}} \\{= {{( {n_{1} - n_{2}} )^{2} - {/( {n_{1} + n_{2}} )^{2}}} = R^{\prime}}}\end{matrix} & {{Equation}\quad (7)} \\\begin{matrix}{T = {( {n_{2}/n_{1}} ) \cdot {t}^{2}}} \\{= {{4 \cdot n_{1} \cdot {n_{2}/( {n_{2} + n_{1}} )^{2}}} = T^{\prime}}} \\{= {t \cdot t^{\prime}}}\end{matrix} & {{Equation}\quad (8)}\end{matrix}$

[0082] The following relational expression also holds.

r+t′=1

r ² +t=1

[0083] On the basis of the above relationships described above, anexplanation will be given of the optical model of an organic EL display.

[0084]FIG. 8 is a conceptual view of the optical model of the organic ELdisplay. All the organic EL devices can be simplified optically asillustrated in FIG. 8.

[0085]FIG. 3 is a specific case of the case shown in FIG. 8. FIG. 3shows the case where the reflectance r₂ on the rear reflecting plane 11is high enough so that it can be considered that r₂ nearly equals tovalue “1” and the reflectance r₁ on the front reflecting plane 12 isr1<<1. The intensity of the light when reflection is repeated twiceormore on the front reflecting plane 12 is negligibly small. Therefore,only the four optical waves 9 a, 9 b, 9 c and 9 d within the device haveonly to be considered.

[0086]FIG. 9 is a view illustrating the multiple reflection of the lightemitted internally in the device and the electric-field amplitude of thelight externally extracted from the device in the configuration of FIG.8. With the electric-field amplitude of the internally emitted lightbeing E, the electric-field amplitude of each of the light waves iswritten.

[0087] The ratio of the amplitude of the synthesized wave of the lightwaves externally extracted to the electric-field amplitude E of theinternally emitted light is referred to as “synthesized electric-fieldamplitude transmittance of the extracted EL light” and is simply denotedby m_(EL).

[0088] Since all the light beams externally extracted have been adjustedto be in phase, simple addition thereof may be made. Thus, the mEL ofthe light wave extracted on the side of a viewer 7 can be expressed asfollows. $\begin{matrix}\begin{matrix}{m_{EL} = {t_{1}\{ {1 + r_{2} + {r_{1}r_{2}} + {r_{1}r_{2}^{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad \}}} \\{= {t_{1}\{ {( {1 + {r_{1}r_{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad ) +} }} \\ {r_{2}( {1 + {r_{1}r_{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad )} \} \\{= {{t_{1}\{ {( {1 + r_{2}} ){\sum\limits_{k = 0}^{\infty}( {r_{1}r_{2}} )^{k}}} \}} = {\frac{1 + r_{2}}{1 - {r_{1}r_{2}}}t_{1}}}}\end{matrix} & {{Equation}\quad (9)}\end{matrix}$

[0089] In the case, an relational expression 0≦|r₁ r₂|≦1 led from0≦|r₁|≦1 and 0≦|r₂|≦1 and the following formulas have been employed.$\begin{matrix}{{{\lim\limits_{n->\infty}{\sum\limits_{k = 0}^{n}p^{k}}} = {{\lim\limits_{n->\infty}\frac{1 - p^{n + 1}}{1 - p}} = \frac{1}{1 - p}}}{{wherein}\quad ( {0 < p < 1} )}} & {{Equation}\quad (10)}\end{matrix}$

[0090] Likewise, the synthesized electric-field transmittance of the ELlight (m_(EL)) extracted on the side opposite to a viewer 7 can beexpressed as follows. $\begin{matrix}\begin{matrix}{m_{EL} = {t_{2}\{ {1 + r_{1} + {r_{1}r_{2}} + {r_{1}^{2}r_{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad \}}} \\{= {t_{2}\{ {( {1 + {r_{1}r_{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad ) +} }} \\ {r_{1}( {1 + {r_{1}r_{2}} + {r_{1}^{2}r_{2}^{2}} + \ldots}\quad )} \} \\{= {{t_{2}\{ {( {1 + r_{1}} ){\sum\limits_{k = 0}^{\infty}( {r_{1}r_{2}} )^{k}}} \}} = {\frac{1 + r_{1}}{1 - {r_{1}r_{2}}}t_{2}}}}\end{matrix} & {{Equation}\quad (11)}\end{matrix}$

[0091]FIG. 10 is a conceptual view of the multiple interferencephenomenon in the organic EL device regarding incident external light 22a. The light on the side of the viewer 7 includes a first reflectedlight beam 22 b and synthesized light beams 23 which are incident to andpermeate through the inside of the device and emerges externally fromthe device while repeating the multiple reflections. In the device inwhich the multiple-reflected light beams of the EL light emittedinternally are adjusted to be in phase, all the synthesized light beams23 are automatically in phase.

[0092] From the Fresnel formulas, the first reflected light beam 22 bprovides the reflectance r₁′ with an inverted sign according to therelationship of refractivity between the medium of the incident side andthe medium of the transmissive side. This means that the phase isshifted by p. Incidentally, the incident light beam 22 a and permeatinglight beam 22 c are always in phase.

[0093]FIGS. 11A and 11B are views showing the inverting status of thephase of the light beam. The case where the phase is inverted is denotedby “−1”, whereas the case whereas the phase is not inverted is denotedby “1”. Since r=−r′, when the reflection accompanying phase inversionoccurs in a certain incidence direction, in the light incidence in theopposite direction, the phase is not inverted.

[0094]FIG. 11A is directed to the case where at the first reflection ofthe external light, its phase is inverted. In this case, the lightwithin the device is reflected in phase on the front reflecting area 20.Specifically, the light which travels back and forth once between thefront reflecting area 20 and a rear reflecting area 19 is adjusted toprovide a phase difference of 2 mn (m denotes a natural number) throughpropagation/reflection. This adjustment is identical to the optimizingof the device as explained already. The device adjusted so as tointensify the EL light is automatically adjusted in this manner.

[0095]FIG. 11B is directed to the case where at the first reflection ofthe external light, its phase is not inverted. In this case, the lightwithin the device is reflected in an opposite phase on the frontreflecting area 20. Specifically, the light which travels back and forthonce between the front reflecting area 20 and a rear reflecting area 19is adjusted to provide a phase difference of 2(m−1)n (m denotes anatural number) through propagation/reflection. The adjustment isidentical to the aforementioned optimizing of the device. The deviceadjusted so as to intensify the EL light is automatically adjusted inthis manner.

[0096] As described above, in the optimized EL device, in any case, inthe first reflected light beam 22 b and the penetrated synthesized lightbeams 23 in FIG. 10 are in opposite phase. The synthesized amplitudes ofthe penetrated synthesized light beams 23 can be expressed as follows.$\begin{matrix}{{t_{1}t_{2}^{\prime}{r_{2}( {1 + {r_{1}r_{2}} + ( {r_{1}r_{2}} )^{2} + \ldots}\quad )}} = {t_{1}t_{1}^{\prime}{r_{2}/( {1 - {r_{1}r_{2}}} )}}} & {{Equation}\quad (12)}\end{matrix}$

[0097] From the Equation (12), the external light synthesized amplitudereflectance m_(EXT) which represents the ratio of the electric fieldamplitude of all the synthesized light waves in FIG. 10 to theelectric-field amplitude A of the incident external light 22 a can beexpressed as follows.

m _(EXT) =|r ₁ ′|−|t ₁ t ₁ ′r ₂/(1−r ₁ r ₂)|  Equation (13)

[0098] By, using the relational expression t₁t₁′=T₁=1−r₁ ² and|r₁′|=|r₁|, Equation (13) can be rewritten as follows. $\begin{matrix}\begin{matrix}{m_{EXT} = {{r_{1}^{\prime}} - {{r_{2}}{( {1 - {r_{1}}^{2}} )/( {1 - {{r_{1}r_{2}}}} )}}}} \\{= {( {{r_{1} - r_{2}}} )/( {1 - {{r_{1}r_{2}}}} )}}\end{matrix} & {{Equation}\quad (14)}\end{matrix}$

[0099] The explanation made hitherto is the general optical modeanalysis of the organic EL device. In summary, using the reflectance r₁,r₁′ and transmittance t₁, t₁′ on the front reflecting area and thereflectance r₂ on the rear reflecting area, assuming that theelectric-field amplitude of the internal EL light is E and theelectric-field amplitude of the incident light is A, the electric-fieldamplitude E_(OUT) of the EL light externally extracted and theelectric-field amplitude A_(REF) of the incident external lightreflected externally can be expressed as follows. $\begin{matrix}{E_{out} = {{m_{EL} \cdot E} = {\frac{1 + r_{2}}{1 - {r_{1}r_{2}}}{t_{1} \cdot E}}}} & {{Equation}\quad (15)} \\{A_{REF} = {{m_{EXT} \cdot A} = {\frac{{r_{1} - r_{2}}}{1 - {{r_{1}r_{2}}}} \cdot A}}} & {{Equation}\quad (16)}\end{matrix}$

[0100] In the case above, the optical intensity I_(OUT) and I_(REF) locan be expressed by the square of the E_(OUT) and A_(REF). Particularly,the condition that A_(REF) equals to 0 is when r₁ equals to r₂.

[0101] Regarding the Equation (15), the calculation result of theintensity ratio M_(EL)=I_(OUT)/E² in the range in which r₁ and r₂ are ina range of from 0 to 0.95 is shown in Table 1. Table 1 shows that theintensity reflectance M_(EL) equals to value 1.00 on the columns on thediagonal line satisfying r₁=r₂. TABLE 1 EL Light Apparent IntensityTransmittance M_(EL) r₁ r₂ 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 0.00 1.00 0.900.81 0.72 0.64 0.56 0.49 0.42 0.36 0.30 0.25 0.20 0.16 0.12 0.09 0.060.04 0.04 0.01 0.00 0.05 1.10 1.00 0.90 0.81 0.72 0.64 0.56 0.48 0.410.35 0.29 0.24 0.19 0.14 0.11 0.07 0.05 0.05 0.01 0.00 0.10 1.21 1.101.00 0.90 0.81 0.72 0.63 0.55 0.47 0.40 0.34 0.27 0.22 0.17 0.13 0.090.06 0.06 0.01 0.00 0.15 1.32 1.21 1.10 1.00 0.90 0.80 0.71 0.62 0.540.46 0.39 0.32 0.26 0.20 0.15 0.10 0.07 0.07 0.02 0.00 0.20 1.44 1.331.21 1.11 1.00 0.90 0.80 0.70 1.61 0.53 0.44 0.37 0.30 0.23 0.18 0.120.08 0.08 0.02 0.01 0.25 1.56 1.45 1.33 1.22 1.11 1.00 0.89 0.79 0.690.60 0.51 0.43 0.35 0.27 0.21 0.15 0.10 0.10 0.03 0.01 0.30 1.69 1.571.45 1.34 1.22 1.11 1.00 0.89 0279 0.68 0.58 0.49 0.40 0.32 0.24 0.180.12 0.12 0.03 0.01 0.35 1.82 1.70 1.59 1.47 1.35 1.23 1.11 1.00 0.890.78 0.67 0.57 0.47 0.37 0.29 0.21 0.14 0.14 0.04 0.01 0.40 1.96 1.841.72 1.60 1.48 1.36 1.24 1.12 1.00 0.88 0.77 0.65 0.54 0.44 0.34 0.250.17 0.17 0.05 0.01 0.45 2.10 1.99 1.87 1.75 1.62 1.50 1.38 1.25 1.131.00 0.88 0.75 0.63 0.51 0.40 0.30 0.21 0.21 0.06 0.02 0.50 2.25 2.142.02 1.90 1.78 1.65 1.53 1.40 1.27 1.13 1.00 0.87 0273 0.60 0.48 0.360.25 0.25 0.07 0.02 0.55 2.40 2.29 2.18 2.06 1.94 1.82 1.69 1.56 1.421.28 1.14 1.00 0.86 0.71 0.57 0.44 0.31 0.31 0.09 0.03 0.60 2.56 2.462.35 2.23 2.12 1.99 1.87 1.73 1.60 1.45 1.31 1.15 1.00 0.84 0.68 0.530.38 0.38 0.12 0.03 0.65 2.72 2.62 2.52 2.41 2.30 2.18 2.06 1.93 1.791.65 1.49 1.34 1.17 1.00 0.82 0.65 0.47 0.47 0.16 0.05 0.70 2.89 2.802.71 2.61 2.50 2.39 2.27 2.14 2.01 1.86 1.71 1.55 1.37 1.19 1.00 0.800.60 0.60 0.21 0.06 0.75 3.06 2.98 2.90 2.81 2.71 2.61 2.50 2.38 2.252.11 1.96 1.80 1.62 1.43 1.22 1.00 0.77 0.77 0.29 0.09 0.80 3.24 3.173.10 3.02 2.94 2.85 2.75 2.64 2.52 2.39 2.25 2.09 1272 1.72 1.51 1.271.00 1.00 0.41 0.14 0.85 3.42 3.37 3.31 3.25 3.18 3.10 3.02 2.93 2.832.72 2.59 2.44 2.28 2.09 1.88 1.63 1.34 1.34 0.62 0.23 0.90 3.61 3.573.53 3.49 3.44 3.38 3.32 3.25 3.17 3.08 2.98 2.87 2.73 2.57 2.37 2.141.84 1.84 1.00 0.43 0.95 3.80 3.78 3.76 3.74 3.71 3.68 3.64 3.61 3.563.51 3.45 3.38 3.29 3.18 3.05 2.88 2.64 2.64 1.81 1.00

[0102] Further, regarding Equation (16), the calculation result of theintensity ratio M_(EXT)=I_(REF)/A² in a range in which r₁ and r₂ arefrom 0 to 0.95, respectively is shown in Table 2. Table 2 shows that theintensity reflectance M_(EXT)=0 on the columns on the diagonal linesatisfying r₁=r₂. TABLE 2 External light Apparent Intensity ReflectanceM_(EXT) r₂ 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.000.0% 0.3% 1.0% 2.3% 4.0% 6.3% 9.0% 12.3% 16.0% 20.3% 25.0% 0.05 0.3%0.0% 0.3% 1.0% 2.3% 4.1% 6.4% 9.3% 12.8% 16.7% 21.3% 0.10 1.0% 0.3% 0.0%0.3% 1.0% 2.4% 4.3% 6.7% 9.8% 13.4% 17.7% 0.15 2.3% 1.0% 0.3% 0.0% 0.3%1.1% 2.5% 4.5% 7.1% 10.4% 14.3% 0.20 4.0% 2.3% 1.0% 0.3% 0.0% 0.3% 1.1%2.6% 4.7% 7.5% 11.1% 0.25 6.3% 4.1% 2.4% 1.1% 0.3% 0.0% 0.3% 1.2% 2.8%5.1% 8.2% 0.30 9.0% 6.4% 4.3% 2.5% 1.1% 0.3% 0.0% 0.3% 1.3% 3.0% 5.5%0.35 12.3% 9.3% 6.7% 4.5% 2.6% 1.2% 0.3% 0.0% 0.3% 1.4% 3.3% 0.40 16.0%12.8% 9.8% 7.1% 4.7% 2.8% 1.3% 0.3% 0.0% 0.4% 1.6% 0.45 20.3% 16.7%13.4% 10.4% 7.5% 5.1% 3.0% 1.4% 0.4% 0.0% 0.4% 0.50 25.0% 21.3% 17.7%14.3% 11.1% 8.2% 5.5% 3.3% 1.6% 0.4% 0.0% 0.55 30.3% 26.4% 22.7% 19.0%15.5% 12.1% 9.0% 6.1% 3.7% 1.8% 0.5% 0.60 36.0% 32.2% 28.3% 24.5% 20.7%17.0% 13.4% 10.0% 6.9% 4.2% 2.0% 0.65 42.3% 38.5% 34.6% 30.7% 26.8%22.8% 18.9% 15.1% 11.4% 8.0% 4.9% 0.70 49.0% 45.4% 41.6% 37.8% 33.8%29.8% 25.6% 21.5% 17.4% 13.3% 9.5% 0.75 56.3% 52.9% 49.4% 45.7% 41.9%37.9% 33.7% 29.4% 25.0% 20.5% 16.0% 0.80 64.0% 61.0% 57.9% 54.6% 51.0%47.3% 43.3% 39.1% 34.6% 29.9% 25.0% 0.85 72.3% 69.8% 67.2% 64.4% 61.3%58.0% 54.5% 50.7% 46.5% 42.0% 37.1% 0.90 81.0% 79.2% 77.3% 75.2% 72.9%70.3% 67.6% 64.5% 61.0% 57.2% 52.9% 0.95 90.3% 89.3% 88.2% 87.0% 85.7%84.3% 82.6% 80.8% 78.7% 76.3% 73.5% r₂ 0.55 0.60 0.65 0.70 0.75 0.800.85 0.90 0.95 0.00 30.3% 36.0% 42.3% 49.0% 56.3% 64.0% 72.3% 81.0%90.3% 0.05 26.4% 32.2% 38.5% 45.4% 52.9% 61.0% 69.8% 79.2% 89.3% 0.1022.7% 28.3% 34.6% 41.6% 49.4% 57.9% 67.2% 77.3% 88.2% 0.15 19.0% 24.5%30.7% 37.8% 45.7% 54.6% 64.4% 75.2% 87.0% 0.20 15.5% 20.7% 26.8% 33.8%41.9% 51.0% 61.3% 72.9% 85.7% 0.25 12.1% 17.0% 22.8% 29.8% 37.9% 47.3%58.0% 70.3% 84.3% 0.30 9.0% 13.4% 18.9% 25.6% 33.7% 43.3% 54.5% 67.6%82.6% 0.35 6.1% 10.0% 15.1% 21.5% 29.4% 39.1% 50.7% 64.5% 80.8% 0.403.7% 6.9% 11.4% 17.4% 25.0% 34.6% 46.5% 61.0% 78.7% 0.45 1.8% 4.2% 8.0%13.3% 20.5% 29.9% 42.0% 57.2% 76.3% 0.50 0.5% 2.0% 4.9% 9.5% 16.0% 25.0%37.1% 52.9% 73.5% 0.55 0.0% 0.6% 2.4% 5.9% 13.6% 19.9% 31.7% 48.0% 70.2%0.60 0.6% 0.0% 0.7% 3.0% 7.4% 14.8% 26.0% 42.5% 66.3% 0.65 2.4% 0.7%0.0% 0.8% 3.8% 9.8% 20.0% 36.3% 61.5% 0.70 5.9% 3.0% 0.8% 0.0% 1.1% 5.2%13.7% 29.2% 55.7% 0.75 11.6% 7.4% 3.8% 1.1% 0.0% 1.6% 7.6% 21.3% 48.4%0.80 19.9% 14.8% 9.8% 5.2% 1.6% 0.0% 2.4% 12.8% 39.1% 0.85 31.7% 26.0%20.0% 13.7% 7.6% 2.4% 0.0% 4.5% 27.0% 0.90 48.0% 42.5% 36.3% 29.2% 21.3%12.8% 4.5% 0.0% 11.9% 0.95 70.2% 66.3% 61.5% 55.7% 48.4% 39.1% 27.0%11.9% 0.0%

[0103] The results shown in Table 2 will be applied to Examples 1 and 2.In Example 2, the front reflecting area reflectance r1 can be estimatedas follows. In Example 2, the planes which can be a highly reflectivefront plane are organic material/ITO/glass interfaces. Particularly,since there is a large difference in the refractivity at the ITO/glassinterface whereas there is a small difference in the refractivity at theorganic material/ITO interface, the ITO/glass interface can be the frontreflecting area. The fact above can be considered reasonable becausethat there has been verified experimentally that the maximum EL lightextracting efficiency is obtained when the distance between theinterface and light emitting layer is adjusted to intensify the EL lightoptically.

[0104] The electric-field amplitude reflectance r₁ at the interfacebetween ITO (refractivity n=1.93) and glass (refractivity n=1.52) isexpressed as follows. $\begin{matrix}\begin{matrix}{r_{2} = {( {n_{ITO} - n_{glass}} )/( {n_{ITO} + n_{glass}} )}} \\{= {( {1.93 - 1.52} )/( {1.93 + 1.52} )}} \\{= 0.119}\end{matrix} & {{Equation}\quad (17)}\end{matrix}$

[0105] Since the reflection/transmission phenomenon with no absorptionis at issue, it can be assumed that t₁=1−r₁.

[0106] The rear reflecting area is the interface between the aluminum,which serves as the metallic electrode and the organic material. Sincethe electric-field amplitude reflectance at the Alq/Al interface is92.5%, if (r₁, r₂)=(0.119, 0.925) is substituted for Equation (9), andthe square is taken, the extraction coefficient M_(EL) of the EL lightintensity can be expressed as follows.

M _(EL)=(m _(EL))²=1.9062=3.63   Equation (18)

[0107] The same calculation is made for Example 1. Since the backsideelectrode is made non-reflective, assuming that (r₁, r₂)=(0.119, 0),M_(EL) is expressed as follows.

M _(EL)=(m _(EL))²=0.8892=0.78   Equation (19)

[0108] As can be seen from FIG. 7, the brightness in Example 1 is 0.23times as large as that in Example 2. This is approximately equal to thecalculation result of the ratio between the extraction coefficientsM_(EL), i.e. 0.78/3.63=0.21.

EXAMPLE 3

[0109] As Example 3, an organic EL device with the measured intensityreflectance R=0.12, i.e. r₂=(0.12)^(1/2)=0.346 was made, like Example 1,by adjusting the thickness of the Alq₃ layer which is the secondtransparent layer of the non-reflective electrode. In the external lightEL light intensity is 154 cd/m² and the brightness ratio to that inExample 2 was 0.48.

[0110] When (r₁, r₂)=(0.119, 0.346) is substituted for Equation (9), andthe square is taken, the extraction coefficient M_(EL) of the EL lightintensity can be expressed as follows.

M _(EL)=(m _(EL))²=1.2372=1.531

[0111] Therefore, its ratio to the calculated value in Example 2 is1.531/3.63=0.422. This value is substantially equivalent to the actualbrightness 0.48. Table 3 shows the measured results in Examples 1 to 3.As can be seen, the calculated value and the measured value are fairlyidentical to each other. TABLE 3 EL Light External Intensity RatioElectric Extracting Intensity Normalized Field Measured by Example 2Reflectance Calculated Value: Calculated Measured r₁ r₂ Value 7.5 mA/crValue Value Example 1 0.119 0 0.78 77 0.215 0.24 Example 2 0.119 0.9253.63 312 1 1 Example 3 0.119 0.346 1.53 154 0.422 0.48

EXAMPLES 4 AND 5

[0112] As Example 4, the non-reflective laminated structure was made onthe same condition as Example 3, and its reflectance was measured. AsExample 4, the EL device having the glass/non-reflective laminatedstructure was made and as Example 5, the EL device havingglass/ITO/non-reflective laminated structure was made. The intensityreflectance in the Examples 4 and 5 were measured.

[0113] In Example 5, the intensity reflectance was about 10%. Therefore,the amplitude reflectance of the non-reflective laminated structure canbe estimated as (0.1)^(1/2)=0.316. In Example 4, the intensityreflectance was about 5%. On the basis of Example 5, the external lightreflectance in Example 4 was calculated by substituting (r₁, r₂)=(0.119,(0.1)^(1/2))=(0.119, 0.316) for Equation (14). Its valueM_(EXT)=(m_(EXT))²=4.19% which is fairly identical to the measured valueabout 5%.

[0114] Table 2 shows that in the organic EL device, (r₁, r₂) givingnon-reflectance to the external light is r₁=r₂. Table 1 shows that atr₁=r₂, the extraction coefficient of the EL light M_(EL)=1.

[0115] Now, in the case of (r₁, r₂)=(0.1, 0.35), M_(EL)=1.59 andM_(EXT)=6.7% can be obtained. It can be seen that the EL efficiencycould be improved 1.96 times as large as M_(EL)=0.81 and M_(EXT)=1.0% inthe case of (r₁, r₂)=(0.1, 0).

EXAMPLE 6

[0116] As for Example 6, a plurality of organic EL devices with anelectrode having intensity reflectance changed, as Example 4, byadjusting the thickness of a thin aluminum layer which is the firsttransparent layer of the non-reflective electrode, is made as Example 1.

[0117]FIG. 12 shows the distribution of brightness efficiency and curvecalculated by Equation (9). As seen from FIG. 12, for various R₂=(r₂)²,the EL intensity of an actual device is fairly identical to thecalculation result in Equation (9).

[0118] As seen from FIG. 12, when the rear intensity reflectanceincreases from 0 to 10%, the brightness increases greatly by 1.79 timesfrom about 70 cd/m² to 125 cd/m². Namely, the device efficiency of theorganic EL device increases approximately twice. In this way, comparedto the case where the rear reflectance is 0, it can be seen that even ifthe rear intensity reflectance increases slightly, the device efficiencyincreases greatly.

[0119] Now referring to Tables 1 and 2, comparison is made between thecase of non-reflection of the external light and the case of slightremaining reflection. As can be seen from Table 2, in the organic ELdevice, (r₁, r₂) which gives non-reflection to the external light isr₁=r₂. As can be seen from Table 1, when r₁=r₂, the extractioncoefficient of the EL light M_(EL) equals to 1 (M_(EL)=1).

[0120] Now, assuming that (r₁, r₂)=(0.1, 0.35) (there is remainingreflection), M_(EL)=1.59, M_(EXT)=6.7%. This shows that the extractionefficiency of the EL light is increased by 1.96 times as compared withM_(EL)=0.81 and M_(EXT)=1.0% in the case where (r₁, r₂)=(0.1, 0). Thus,it can be understood that where there is the slight remainingreflection, the extraction efficiency of the EL light is greatlyimproved as compared with the case of non-reflection of the externallight.

[0121] It is desired that the intensity reflection coefficient R2 in therear reflection area is adjusted so that the external light intensityreflectance M_(EXT) in the display seen from a viewer is 10% or lessowing to optical interference effect. It is preferred that the intensityreflectance R₂ is adjusted in a range of from 5% to 50% in order tointensify the efficiency of externally extracting the EL light. When theexternal light intensity reflectance M_(EXT) in the display seen from aviewer exceeds 10%, the increase in the external light reflectance playsa greater role than the increase in the extraction efficiency of theorganic EL light, thereby deteriorating the visibility of the display.

[0122] The combinations of (r₁, r₂) which satisfies the condition thatthe external light intensity reflectance M_(EXT) in the display seenfrom a viewer is 10% or less are the numerical values within the rangein the vicinity of the diagonal line which can be drawn from the upperleft corner to the lower right corner in Table 2. On the basis ofEquation (16), this range can be expressed by the following Equations(20) and (21). $\begin{matrix}{( \frac{r_{1} - r_{2}}{1 - {r_{1}r_{2}}} )^{2} \leq 0.1} & {{Equation}\quad (20)} \\{( \frac{\sqrt{R_{1}} - \sqrt{R_{2}}}{1 - \sqrt{R_{1}R_{2}}} )^{2} \leq 0.1} & {{Equation}\quad (21)}\end{matrix}$

[0123] When the numerical values within this range are selected toimprove the intensity transmittance M_(EL) of the EL light withreference to Table 1, the display with the reflectance of the externallight suppressed and the light emitting intensity of the EL lightincreased can be acquired. Preferably, among the parameters located onthe lower left side of the diagonal line which can be drawn from theupper left corner to the lower right corner in Table 1, the parametersmay be selected which provide the external light intensity reflectanceM_(EXT) of 10% or less. Referring to Table 1, this range can beexpressed by the following Equations (22) and (23).

r₁≦r₂   Equation (22)

R₁≦R₂   Equation (23)

[0124] In summary, the display is preferably adjusted so that theexternal light intensity reflectance M_(EXT) is 10% or less, andparameters r₁ and r₂, or R₁ and R₂ satisfy the condition: r₁≦r₂, i.e.R₁≦R₂. Such a display which satisfies the above condition can be thedisplay with the reflectance of the external light suppressed and thelight emitting intensityof the EL light increased. In thisconfiguration, the light emitting intensity of the organic EL displaycan be increased in the state where the current flowing through theorganic layer has been decreased.

FIRST EMBODIMENT

[0125] Now referring to FIG. 13, an explanation will be given of a firstembodiment according to the invention.

[0126]FIG. 13 is a view showing an organic EL device 30 according to thefirst embodiment. The organic EL device 30 includes a glass substrate31, a transparent electrode 32 arranged on the glass substrate 31, anorganic layer 34 consisting of a plurality of layers successivelystacked on the transparent electrode 32 and a backside electrode 33stacked on the organic layer 34. Application of a voltage between thetransparent electrode 32 and the backside electrode 33 injects positiveand negative carriers into the device, and the re-combination of thesecarriers generates the electroluminescence within the organic layer 34.Reference numeral 35 denotes one of light emitting sources which emitthe light in the organic layer 34 when the excited organic moleculesreturns to the ground state, and has a size corresponding toapproximately a single molecule. The light emitting sources 35 aredistributed infinitely as a layer. The luminescent light emitted fromthe infinite number of light emitting sources 35 are emitted directly orby multiple reflection within the device toward a viewer 36 through thetransparent electrode 32 and the glass substrate 31.

[0127] In the first embodiment, between the interface between thebackside electrode 33 and the organic layer 34, and the light emittingintensity peak position of the light emitting source 35, and between theinterface between the transparent electrode 32 and the substrate 31, andthe light emitting intensity peak position of the light emitting source35, optical distances are selected so as to increase all themultiply-reflected light beams within the device.

[0128] In the first embodiment, the transparent electrode 32 is made ofindium tin oxide (ITO). However, the transparent electrode 32 may bemade of other transparent conductive films.

[0129] The electric field amplitude reflectance r₁ at the interfacebetween the transparent electrode 32 and the glass substrate 31 can beestimated at 0.119. The electric filed amplitude reflectance at theinterface between the transparent electrode 32 and the organic layer 34of 0.043, which is smaller than 0.119, is negligible. The electric fieldamplitude reflectance r₂ at the interface between the organic layer 28and the backside electrode 27 can be estimated at 0.346. In this case,the measured M_(EL) and M_(EXT) are 1.531 and 3.63%. In a case where r₂equals 0, since M_(EL) is approximately 0.8, the brightness of the ELlight emitting intensity is increased approximately twice. In the firstembodiment, the condition for improving the extraction coefficientM_(EL) of the EL light is r₁≦r₂ from Table 1 and 2. In addition, when r₂is 0.4 or less, the device with the external light intensity M_(EXT) of10% or less can be obtained.

[0130] In the first embodiment, in a case where the external lightintensity M_(EXT) is adjusted to 10% or less, particularly,approximately 0, the organic EL device itself becomes a non-reflectorhaving a non-reflecting structure. In this way, in the first embodiment,using the reflecting plane formed on the side nearer than the lightemitting layer to the viewer and the reflecting plane formed on the sidefarther than the light emitting layer from the viewer, a low reflectingstructure or non-reflecting structure has been realized.

SECOND EMBODIMENT

[0131] The first embodiment relates to the case where the EL light isextracted toward the substrate. The above contemplation applies, as itis, to a top emission type EL organic EL device which extracts the ELlight toward the side opposite to the substrate. FIG. 14 is a viewshowing the top emission type EL organic EL device. Referring to FIG.14, a detailed explanation will be given of the top emission type ELorganic EL device.

[0132] The top emission type organic EL device 40 includes a substrate41, a backside electrode 42 arranged on the substrate 41, an organiclayer 44 consisting of a plurality of layers successively stacked on thebackside electrode 42 and a transparent electrode 43 stacked on theorganic layer 44. The transparent electrode 43 may be a laminatedstructure consisting of a transparent metal and a conductive oxide film.The transparent electrode 43 is made of sputtered ITO.

[0133] The electric-field amplitude reflectance r₁ on the frontelectrode when the EL light is extracted from the transparent electrode43 into the air is expressed by the following equation. $\begin{matrix}{r_{1} = {( {n_{ITO} - n_{air}} )/( {n_{ITO} + n_{air}} )}} \\{= {( {1.93 - 1.0} )/( {1.93 + 1.0} )}} \\{= 0.317}\end{matrix}$

[0134] Table 4 shows the relationship among the electric-field amplitudereflectance r₂ and intensity reflectance R₂ on the rear (substrate side)and external intensity reflectance M_(EXT) and the EL intensityextraction coefficient MEL. TABLE 4 r₂ R₂ M_(EXT) M_(EL) 0 0 0.100 0.4070.317 0.100 0.0 1.00 0.575 0.331 0.1 1.73

[0135] The condition for setting the external light intensityreflectance M_(EXT) at 10% can be solved noting the sign of Equation(14). This corresponds to R₂=0% and 33.1%. In a case above, by applyingthe condition R₁=R₂, the EL light intensity extraction coefficient canbe set at 1.73. This value is equal to 4.25 times as large as in thecase of the intensity reflectance R₂=0% at the backside electrode.

MODIFIED EMBODIMENT

[0136] In the top emission type organic EL device, as shown in FIG. 15,the backside electrode may be a laminated structure consisting of atransparent conductive film and a lowly-reflective film.

[0137] In other words, in the transparent type organic EL device withboth electrodes being transparent, also where an absorptive film isapplied on the rear plane opposite to the light extracting side, r₂ canbe taken as the reflectance.

[0138] In the top emission type organic EL device, as shown in FIGS. 16and 17, the light extracting electrode may be a laminated structureconsisting of a transparent conductive film and semi-transparent film.Equation (14) relates to the case where the absorption of energy at thefirst reflecting plane from the external light is zero. However, anotherabsorption layer may be formed at the interface for adjusting t₁independently from r₁.

[0139] As the device format similar to Example 1, as shown in FIG. 18,the backside electrode may be formed of a laminated structure consistingof a transparent conductive film and a lowly-reflective film.

[0140] As shown in FIG. 19, where there are a plurality of reflectingplanes on the rear surface or front surface, the sum of the light beamsfrom all the reflecting planes located at the rear surface or frontsurface may be defined as the reflected light, and the change in theelectric field amplitude may be defined as r₁ or r₂.

[0141] As shown in FIG. 20A, the structure of the non-reflector (i.e.laminated optical structure with a low reflectance and a lowtransmittance) may be an organic EL device itself. In FIG. 20, atransparent electrode 51, which is made of a semi-transparent film, islocated on the substrate. A semi-transparent organic EL layer 52 havingat least a light emitting layer is stacked on the organic EL layer 52. Areflective backside electrode 53 is stacked on the organic EL layer 52.

[0142] In above arrangement, the transparent electrode 51, organic ELlayer 52 and backside electrode 53 constitute a non-reflector which isadjusted to cancel the external light incident from the side of thesubstrate by its optical interference with the reflected light beams atthe interfaces between the adjacent layers. In such an organic ELdevice, r1 and r2 may be adjusted to set the external light intensityreflectance M_(EXT) at 10% or less.

[0143] It is needless to say that the above arrangement can be appliedto the top emission type organic EL device in such a manner that theelectrode 51 arranged on the substrate is a reflective backsideelectrode and the electrode 53 is a semi-transparent electrode.

[0144] In this way, in a case where the organic EL device itself isconstituted as the non-reflector, it is not necessary to provideseparately a semi-transparent member or optical reflective film so thatthe organic EL device can be low-profiled. Since other members are notrequired, the manufacturing process can be simplified and the productionefficiency can be improved.

[0145] As shown in FIG. 20B, the structure of the non-reflector (i.e.laminated optical structure with a low reflectance and a lowtransmittance) may include a portion of the organic EL device so thatthe one end of the non-reflector is formed of a portion of the organicEL device. In FIG. 20B, a transparent electrode 61, which is made of asemi-transparent film, is located on the substrate. A transparentorganic EL layer 62 having at least a light emitting layer is stacked onthe transparent electrode 61. The organic EL layer 62 incorporates anoptical reflective film 63 having the property of charge transportationand semi-transmittance. A reflective backside electrode 64 is stacked onthe organic EL layer 62.

[0146] In above arrangement, the optical reflective film 63 and backsideelectrode 64 constitute a non-reflector which is adjusted to cancel theexternal light incident from the side of the substrate by its opticalinterference with the reflected light beams at the interfaces betweenthe adjacent layers. In above arrangement, the reflected light at theinterface between the substrate and the transparent electrode 61 and thereflected light at the interface between the transparent electrode 61and organic EL device, which have a very small amount, are negligible.In such an organic EL device, r₁ and r₂ may be adjusted to set theexternal light intensity reflectance M_(EXT) at 10% or less.

[0147] Since the non-reflector has a part of the organic EL device, thenumber of the semi-transparent optical members and optical reflectivefilms to be formed can be reduced so that the organic EL device can below-profiled. Since other members are not required, the manufacturingprocess can be simplified and the production efficiency can be improved.

[0148] In accordance with the organic EL display described hitherto, thelight emission efficiency of the organic EL light can be enhancedwithout using a circularly-polarizing filter while improving thecontrast of the organic EL display. Further, the light emissionefficiency of the organic EL display can be enhanced in the state wherethe current flowing through the organic layer has been decreased.

[0149] Although the present invention has been shown and described withreference to specificpreferred embodiments, various changes andmodifications will be apparent to those skilled in the art from theteachings herein. Such changes and modifications as are obvious aredeemed to come within the spirit, scope and contemplation of theinvention as defined in the appended claims.

What is claimed is:
 1. An organic electroluminescent display comprising:an organic electroluminescent device having a light emitting layer madeof an organic material and at least two electrodes sandwiching the lightemitting layer; a front reflecting portion arranged on a side of aviewer with respect to the light emitting layer; and a rear reflectingportion arranged on a side opposite to the viewer with respect to thelight emitting layer, wherein the optical film thickness of the lightemitting layer, intensity reflectance R₁ at the front reflecting portionand intensity reflectance R₂ at the rear reflecting portion areconfigured so that an intensity reflectance of the external light viewedfrom the viewer is set to be 10% or less by an optical interferenceeffect.
 2. The organic electroluminescent display as claimed in claim 1,wherein the intensity reflectance R₁ and the intensity reflectance R₂are configured to be R₁≦R₂.
 3. The organic electroluminescent display asclaimed in claim 1, wherein the intensity reflectance R₁ and theintensity reflectance R₂ are configured to satisfy the followingEquation (1). $\begin{matrix}{( \frac{\sqrt{R_{1}} - \sqrt{R_{2}}}{1 - \sqrt{R_{1}R_{2}}} )^{2} \leq 0.1} & {{Equation}\quad (1)}\end{matrix}$


4. The organic electroluminescent display as claimed in claim 1, whereinthe intensity reflectance R₁ and the intensity reflectance R₂areconfigured to be approximately equal.
 5. The organic electroluminescentdisplay as claimed in claim 1, wherein the intensity reflectance R₂ isconfigured to be in a range of from 5% to 50%.
 6. The organicelectroluminescent display as claimed in claim 1, wherein the frontreflecting portion comprises a substrate and at least one transparent orsemi-transparent film.
 7. The organic electroluminescent display asclaimed in claim 6, wherein the film comprises either one of the twoelectrodes.
 8. The organic electroluminescent display as claimed inclaim 1, wherein the organic electroluminescent device further comprisesa substrate, and wherein the front reflecting portion comprises aninterface between either one of the electrodes and the substrate of theorganic electroluminescent device.
 9. The organic electroluminescentdisplay as claimed in claim 1, wherein the organic electroluminescentdevice further comprises a transparent film, and wherein the frontreflecting portion comprises an interface between either one of theelectrodes and the transparent film of the organic electroluminescentdevice.
 10. The organic electroluminescent display as claimed in claim1, wherein the front reflecting portion comprises air and a transparentof semi-transparent film.
 11. The organic electroluminescent display asclaimed in claim 1, wherein the rear reflecting portion comprises eitherone of the electrodes.
 12. The organic electroluminescent display asclaimed in claim 1, wherein the rear reflecting portion comprises aplurality of reflective, transparent of semi-transparent films.
 13. Theorganic electroluminescent display as claimed in claim 12, wherein atleast one of the films comprises either one of the electrodes.
 14. Anorganic electroluminescent display equipped with an organicelectroluminescent device having a laminated optical structure with lowreflectance and transmittance, wherein the organic electroluminescentdevice comprises: a substrate; a first semi-transparent film; a secondsemi-transparent film; and a reflective film, and wherein the firstsemi-transparent film, the second semi-transparent film and thereflective film are laminated on the substrate in this order or an orderopposite thereto.
 15. The organic electroluminescent display as claimedin claim 14, wherein the first semi-transparent film and the reflectivefilm comprises an electrode, respectively, and wherein the secondsemi-transparent film comprises a light emitting layer made of anorganic material.
 16. An organic electroluminescent display comprising:a laminated optical structure with low reflectance and transmittance;and an organic electroluminescent device, wherein the organicelectroluminescent device comprises: an organic electroluminescent layerhaving a light emitting layer made of an organic material and atransporting layer configured to transport charges to the light emittinglayer, and two electrodes configured to sandwich the organicelectroluminescent layer, wherein the laminated optical structurecomprises at least two layers, and wherein the laminated opticalstructure comprises the transporting layer.